In recent years, a design rule of a semiconductor device that constitutes a Large Scale Integrated (LSI) circuit has been increasingly shrinking from a requirement for higher integration and higher operational speed of the LSI. When a large number of semiconductor devices are formed in a chip, each of the semiconductor devices needs to be electrically isolated so that the semiconductor devices do not adversely affect one another. As a device isolation technology for forming a device isolation structure, a Sallow Trench Isolation (STI) process has been known. In the STI process, a groove (trench) is formed by anisotropically etching an upper surface of a silicon substrate (semiconductor wafer); the groove is filled with an insulating material such as silicon oxide; and the insulating material is planarized, so that the semiconductor devices are isolated with the insulating material so formed. The STI process is advantageous in that only a small area is needed, which enables further miniaturization, compared to a Local Oxidation of Silicon (LOCOS) process.
Referring to FIG. 1, a trench forming step in the STI process is explained. First, after a thin insulating film such as a silicon oxide (SiO2) film or a silicon nitride (SiN) film is formed on a silicon substrate (a semiconductor wafer) 211, the insulating film is patterned by photolithography and etching, as shown in FIG. 1(a), thereby obtaining an etching mask 212 to be used for etching the semiconductor wafer 211. Next, the semiconductor wafer 211 is etched using the etching mask 212, as shown in FIG. 1(b), thereby forming a shallow trench.
In this etching process, generally, an etching gas is activated by plasma, and the semiconductor wafer 211 on which the etching mask 212 is formed is exposed to the activated etching gas, so that a predetermined pattern is formed.
While there are an Electron Cyclotron Resonance method and a parallel plate method as a method for generating plasma, a microwave plasma apparatus employing a microwave plasma method that uses microwaves to generate high density plasma is widely used because plasma can be stably generated even in a relatively low pressure vacuum environment of 0.1 mTorr (13.3 mPa) to several tens mTorr (several Pa) by the microwave plasma method. Specifically, a plasma etching apparatus employing a Radial Line Slot Antenna (RLSA) microwave plasma method is used, because the electron temperature is low despite the high plasma density and plasma density uniformity being excellent, so that uniform etching is realized while reducing damage on the substrate to be etched (See Patent Document 1, for example).
In this case, when high frequency electric power having a radio frequency (RF) is applied to a susceptor on which the semiconductor wafer is placed as alternating bias electric power, if needed, the ions generated by the plasma can be pulled toward the upper surface of the semiconductor wafer, thereby efficiently performing the etching (See Patent Document 2, for example).
Patent Document 1: International Publication Pamphlet No. 06/064898.
Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2006-156675.